Long-term interactive effects of N addition with P and K availability on N status of Sphagnum

Little information exists concerning the long-term interactive effect of nitrogen (N) addition with phosphorus (P) and potassium (K) on Sphagnum N status. This study was conducted as part of a long-term N manipulation on Whim bog in south Scotland to evaluate the long-term alleviation effects of phosphorus (P) and potassium (K) on N saturation of Sphagnum (S. capillifolium). On this ombrotrophic peatland, where ambient deposition was 8 kg N ha^-1 yr^-1, 56 kg N ha^-1 yr^-1 of either ammonium (NH₄⁺, N_red) or nitrate (NO₃^-, N_ox) with and without P and K, were added over 11 years. Nutrient concentrations of Sphagnum stem and capitulum, and pore water quality of the Sphagnum layer were assessed. The N-saturated Sphagnum caused by long-term (11 years) and high doses (56 kg N ha^-1 yr^-1) of reduced N was not completely ameliorated by P and K addition; N concentrations in Sphagnum capitula for N_red 56 PK were comparable with those for N_red 56, although N concentrations in Sphagnum stems for N_red 56 PK were lower than those for N_red 56. While dissolved inorganic nitrogen (DIN) concentrations in pore water for N_red 56 PK were not different from N_red 56, they were lower for N_ox 56 PK than for N_ox 56 whose stage of N saturation had not advanced compared to N_red 56. These results indicate that increasing P and K availability has only a limited amelioration effect on the N assimilation of Sphagnum at an advanced stage of N saturation. This study concluded that over the long-term P and K additions will not offset the N saturation of Sphagnum.